Nuclear magnetic resonance (NMR) is a phenomenon exhibited by a select group of atomic nuclei and is based upon the existence of nuclear magnetic moments in these nuclei (termed "NMR active" nuclei). When an NMR active nucleus is placed in a strong, uniform and steady magnetic field, it processes at a natural resonance frequency known as a Larmor frequency, which is characteristic of each nuclear type and is proportional to the applied field strength in the location of the nucleus. Typical NMR active nuclei include .sup.1 H (protons), .sup.13 C, .sup.19 F and .sup.31 P. The resonant frequencies of the nuclei can be observed by monitoring with an RF receiver the transverse magnetization which results after a strong RF pulse. It is common practice to convert the measured signal to a frequency spectrum by means of Fourier transformation.
In order to use the NMR phenomenon to obtain an image of a sample, a magnetic field is applied to the sample, along with a magnetic field gradient which depends on physical position so that the field strength at different sample locations differs. When a field gradient is introduced, as previously mentioned, since the Larmor frequency for a particular nuclear type is proportional to the applied field, the Larmor frequencies of the same nuclear type will vary across the sample and the frequency variance will depend on physical position. By suitably shaping the applied magnetic field and processing the resulting NMR signals for a single nuclear type, a nuclear spin density image of the sample can be developed. Because the NMR signal which is developed is a function of the total number of nuclei of a given type, it is common to use a nucleus which is found in abundance in the sample to be imaged. For example, .sup.1 H (protons) are commonly used because they are abundant in many materials and therefore, generate a large NMR signal.
The Larmor frequency for a given nuclear type is not exactly proportional to the applied magnetic field, since other interactions can also affect the Larmor frequency. A complex spectrum of frequencies is typically observed in the absence of a magnetic field gradient. Because the Larmor frequency dependence on nuclear position is the basis for forming an image, in order to resolve two spatially-distinct but physically-adjacent elements, the magnetic field gradient strength must be increased to a point where the resonance frequency of nuclei in each element are shifted from each by an amount which is greater than the natural spread in resonance frequencies.
Aside from a practical limit on magnetic field gradient strength which can be generated with existing equipment, increasing the gradient strength also broadens the total spread of resonant frequencies over the entire sample width. As all of these frequencies must be accommodated by the RF receiver, the bandwidth of the receiver must also be increased. Noise enters the receiver in proportion to the square-root of the receiver bandwidth so that as the magnetic gradient strength increases, additional redundant measurements must be taken to extract the signal from the noise. Since the redundant measurements require extra time, the amount of time needed to acquire an image therefore also places a practical limit on the image resolution. Consequently, most prior art techniques for increasing image resolution have attempted to reduce the resonance line width as much as possible rather than increasing the magnetic field gradient.
The natural resonance line width in a sample is generally most influenced by three factors: dipolar couplings, chemical shifts and susceptibility broadening. The most serious of these broadening factors is the dipolar coupling. More particularly, the magnetic moments in neighboring nuclei perturb each other, resulting in interactions called dipole-dipole couplings. If the neighboring nuclei are of the same type, the perturbations are called homonuclear dipolar couplings and tend to broaden the characteristic resonance lines and reduce image resolution. In liquids, the field perturbations induced by dipolar couplings are time-averaged to zero and thus do not severely affect image resolution, but in solids, these couplings can give rise to very large static magnetic field components which can be as much as several Gauss for interacting protons. A field perturbation of this magnitude can significantly widen a resonance peak and reduce image resolution.
Chemical shifts are also an important source of line broadening. More particularly, although identical nuclei have the same frequency dependence upon the magnetic field, differences in the chemical environment of each nucleus can modify the applied magnetic field in the local vicinity of the nucleus, so that nuclei in the same sample do not experience the same net magnetic field. The differences in the local magnetic field result in slight spectral shifts in the Larmor frequencies between two such chemically non-equivalent nuclei, called "chemical shifts" which tend to broaden the resonance peaks and reduce image resolution.
The chemical shifts have a component which is anisotropic in that it depends on the particular orientation of a molecule to the applied Zeeman field and an isotropic part which is independent of the applied field direction. In liquids, the rapid molecular motion tends to average out the anisotropic parts of the chemical shifts leaving relatively small isotropic parts. However, in solids, the orientation of the solid molecules is relatively fixed with respect to the applied Zeeman field and, accordingly, the anisotropic chemical shift components do not average to zero, resulting in a much more severe peak broadening problem.
Susceptibility broadening occurs because an assumption is made that magnetic susceptibility of the nuclei is the same across the entire sample and that the applied magnetic field gradient is constant across the entire sample. This is practically not true in either case and each sample and applied field will have inhomogeneous areas which give rise to additional broadenings.
Therefore, in solids imaging systems, it is important to suppress homonuclear dipolar couplings, chemical shifts and susceptibility broadenings (chemical shifts and susceptibility broadenings are referred to collectively as "inhomogeneous" broadenings) in order to obtain high resolution without increasing the magnetic field gradient. One prior art method of reducing some of the aforementioned broadenings consists of orienting the solid sample at the "magic angle" (54.degree. 44') with respect to the applied Zeeman field and physically rotating the solid at a relatively rapid rate thereby causing the perturbing components to average to zero and to greatly reduce the resulting perturbations. This technique is called "magic angle sample spinning" or MASS. In this case, the magnetic field gradient must also rotate in synchronism with the rotating sample.
Other prior art techniques take advantage of the fact that, since the orientation of the sample molecules with respect to the applied field is relatively fixed in a solid, after an initial excitation, the nuclear spins change or evolve in time in known ways (which can be described mathematically by means of a mathematical operator known as a "Hamiltonian" operator that depends on various factors including the dipolar coupling, chemical shift and susceptibility factors). Although the nuclear evolution produced by the field gradient and inhomogeneous perturbations have the same spin dependence, the inhomogeneous perturbations are time independent. Therefore, it is possible to differentiate the two types of interactions by making the magnetic field gradient strength time-dependent. Accordingly, some conventional imaging schemes use pulsed gradients or oscillating gradients. Still other prior art two-dimensional imaging schemes step the gradient strength in a controlled fashion between successive data points.
Further known techniques take advantage of the fact that the evolution caused by inhomogeneous perturbations is time-independent. These latter techniques involve irradiating the sample nuclei with selected radio-frequency (RF) pulses at, or near, the Larmor frequency. The various orientations and phases of the RF pulses are selected to periodically reverse the nuclear evolution due to dipolar, chemical shift and susceptibility factors (including any inhomogeneity in the applied static field) in order to effectively average out the spin interactions over time so that their contribution to the final output is greatly diminished. As this periodic reversal would also average and destroy imaging information if the gradient was time independent, the imaging information is reintroduced by making the amplitude of the magnetic field gradient time dependent so that, over time, the nuclear spin evolution resulting from the undesired factors averages to zero, but the nuclear spin evolution due to the magnetic field gradient does not average to zero. The result is that the Larmor resonance line is effectively "narrowed" in frequency, thereby increasing image resolution. An example of this latter technique is discussed in detail in ".sup.1 H-Refocussed Gradient Imaging of Solids", J. B. Miller and A. N. Garroway, Journal of Magnetic Resonance, v. 82, pp. 529-538.
In other known techniques, this latter pulsing technique is combined with the aforementioned sample spinning in a technique called CRAMPS (Combined Rotation And Multiple Pulse Spectroscopy).
Although the aforementioned RF pulse sequences are effective in narrowing the resonance lines, in practice, they are difficult to implement since they generally require special instrumentation and a high degree of technical skill. In particular, the methods are highly susceptible to interference due to inhomogeneous RF fields, pulse imperfections and transmitter misadjustments. Consequently, very precise and sophisticated NMR instruments must be used and great care must be taken to properly adjust the instruments during use. The result is that a large amount of time is necessary to construct even simple images.
Accordingly, it is an object of the present invention to provide a method for increasing the resolution of an NMR solids imaging system.
It is another object of the present invention to provide a method for increasing the resolution of an NMR solids imaging system which does not require special instrumentation or experimental skills.
It is yet another object of the present invention to provide a method for increasing the resolution of an NMR solids imaging system which is tolerant of RF field inhomogeneity, pulse imperfections, and transmitter misalignment.
It is still another object of the present invention to provide a method for increasing the resolution of an NMR solids imaging system by narrowing the resonance line width.
It is a further object of the present invention to provide a method for increasing the resolution of an NMR solids imaging system which increases resolution by utilizing a line-narrowing multiple-pulse RF pulse sequence.
It is yet a further object of the present invention to provide a method for increasing the resolution of an NMR solids imaging system in which a multiple-pulse RF pulse sequence time averages to zero the nuclear evolution due to homonuclear dipolar couplings as well as time independent inhomogeneous broadenings.